The subject matter disclosed herein relates generally to voltage switching systems, and more particularly, to methods and apparatus for voltage switching in imaging systems, such as diagnostic x-ray imaging systems.
In conventional computed tomography (CT) x-ray imaging systems, an x-ray source emits a cone-shaped beam of x-rays toward a subject or object, such as a patient. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the x-ray intensity received by that detector element. The electrical signals are quantized and transmitted to a data processing system for analysis, which ultimately produces an image.
CT imaging systems may comprise energy-discriminating (ED), multi-energy (ME), and/or dual-energy (DECT) imaging systems that may be referred to as an EDCT, MECT, and/or DECT imaging system. The EDCT, MECT, and/or DECT imaging systems are configured to measure energy-sensitive projection data. The energy-sensitive projection data may be acquired using multiple applied x-ray spectra by modifying the operating voltage of the x-ray tube or utilizing x-ray beam filtering techniques (e.g., energy-sensitive x-ray generation techniques), or by energy-sensitive data acquisition by the detector using energy-discriminating, or with photon counting detectors or dual-layered detectors (e.g., energy-sensitive x-ray detection techniques).
With x-ray generation techniques, various system configurations utilize modification of the operating voltage of the x-ray tube including: (1) acquisition of low-energy and high-energy projection data from two sequential scans of the object using different operating voltages of the x-ray tube, (2) acquisition of projection data utilizing rapid switching of the operating voltage of the x-ray tube to acquire low-energy and high-energy information for an alternating subset of projection views, or (3) concurrent acquisition of energy-sensitive information using multiple imaging systems with different operating voltages of the x-ray tube.
EDCT/MECT/DECT provides energy discrimination capability that allows material characterization. For example, in the absence of object scatter, the system utilizes signals from two applied photon spectra, namely the low-energy and the high-energy incident x-ray spectrum. The low-energy and high-energy incident x-ray spectra are typically characterized by the mean energies of the applied x-ray beams. For example, the low-energy x-ray spectrum comprises x-ray photons with lower-energy photons, resulting in a lower mean energy, relative to the high-energy x-ray spectrum. The detected signals from low-energy and high-energy x-ray spectra, either from two different applied spectra (x-ray generation techniques) or by regions of the same applied spectrum (x-ray detection techniques) provide sufficient information to estimate the effective atomic number of the material being imaged. Typically, x-ray attenuation mechanisms (Compton scattering or Photoelectric absorption) or the energy-sensitive attenuation properties of two basis materials (typically water and calcium for patient scanning) are used to enable estimation of the effective atomic number.
Dual-energy scanning can obtain diagnostic CT images that enhance contrast separation within the image by utilizing energy-sensitive measurements. To facilitate processing of the energy-sensitive measurements, the applied x-ray spectrum should be constant during an integration period. For example, such CT systems that acquire interleaved subsets of low-energy and high-energy projection data (versus two separate scans) should operate to maintain the accelerating voltage steady during an acquisition interval. Also, the change from one voltage level to another voltage level should occur very rapidly. Less stable x-ray tube operating voltages and/or slower operating voltage switching times result in a reduction in the difference in effective mean energy (the average of the mean energies of time-varying x-ray spectra) of the applied x-ray spectra, which reduces the fidelity of the system in characterizing different materials.
Thus, while switching the x-ray tube potential (voltage), for example, by using high-frequency generators, may solve some of the problems related to conventional dual-energy scanning (acquiring energy-sensitive projection data on alternate scans of the object), such a configuration does not always provide the switching speed needed for certain imaging applications. For example, cardiac imaging cannot be effectively performed by simply switching the x-ray source potential between two sequential scans of the human thorax due to cardiac motion. Furthermore, for systems utilizing rapid switching of the x-ray potential for subsets of projection angles, the switching speed of the x-ray tube potential may not be sufficient for the fast gantry rotation speeds required to freeze motion for cardiac imaging. There is often a delay in the response time of the switched operating potential between the high-frequency generator and the x-ray tube, due in part to the capacitance of the cable connecting the device and the x-ray tube.
The delay in response time is dependent on the x-ray beam current of the x-ray tube as the beam current also either helps or hinders the discharge of the associated capacitance. Accordingly, the rise time in switching the generator from a first (low) voltage, or low kVp, level to a second (high), or high kVp, level is limited by the power of the high-voltage generator, which may be suboptimal for dual-energy imaging in many medical applications. Similarly, the fall time between switching the high kVp to a low kVp level is generally very slow due to the need to discharge the system capacitance, which effectively reduces the energy separation of the applied spectra, resulting in reduced material specification sensitivity and, therefore, the effectiveness of the dual-energy imaging. As such, these insufficient switching speeds often lead to projection data pair mis-registration and streak artifacts in reconstructed images. Additionally, many industrial CT systems for baggage inspection utilize stationary anode tube configurations with x-ray beam currents that are an order of magnitude or more lower than x-ray beam currents used with medical CT system employing rotating-anode technology. As such, the time required to switch the operating voltage of the x-ray tube is prohibitively long.
For radiographic x-ray imaging systems, the limitations mentioned above also apply. Radiographic x-ray systems acquire one or more views of the imaged object, which may be presented as two-dimensional projection images, or in some cases where several more projection data are acquired, as three-dimensional images generated using tomosynthesis techniques. The aforementioned limitations regarding switching speed apply to x-ray radiographic or tomosynthesis systems, such as due to the capacitance of the high-voltage cable connecting the generator to the x-ray tube, the x-ray tube capacitance itself, the power of the generator, and the x-ray beam current, which may all limit switching speed. The capacitance of the high-voltage cable and the x-ray tube are considered the system or main capacitance.